Environmentally Relevant Chronic Low-Dose Tritium and Gamma Exposures do not Increase Somatic Intrachromosomal Recombination in pKZ1 Mouse Spleen.

The toxicity of tritium is a public health concern given its presence and mobility in the environment. For risk predictions using radiological protection models, it is essential to allocate an appropriate radiation weighting factor (WR). This in turn should be consistent with the observed relative biological effectiveness (RBE) of tritium beta radiation. Although the International Commission on Radiological Protection (ICRP) currently recommends a WR of 1 for the calculation of committed effective dose for X rays, gamma rays and electrons of all energies, including tritium energies, there are concerns that tritium health risks are underestimated and that current regulatory tritium drinking water standards need revision. In this study, we investigated potential cytotoxic and genotoxic effects in mouse spleen after one month and eight months of chronic exposure to low-dose tritiated water (HTO). The dose regimes studied were designed to mimic human chronic consumption of HTO at levels of 10 kBq/l, 1 MBq/l and 20 MBq/l. The total doses from these radiation exposures ranged from 0.01 to 180 mGy. We also compared the biological effects of exposure to HTO with equivalent exposure to external whole-body 60Co gamma rays. Changes in spleen weight and somatic intrachromosomal recombination (DNA inversions) in spleen tissue of pKZ1Tg/+ mice were monitored. Our results showed no overall changes in either spleen organ weights and no increase mouse splenic intrachromosomal recombination frequencies, indicating that current drinking water standards for tritium exposure in the form of HTO are likely to be adequately protective against cytotoxic and genotoxic damage in spleen. These results demonstrate no evidence for cytotoxicity or genotoxicity in mouse spleen following chronic exposures to HTO activities (or equivalent gamma doses) up to 20 MBq/L.

Histone H2AX Is Involved in FoxO3a-Mediated Transcriptional Responses to Ionizing Radiation to Maintain Genome Stability.

Histone H2AX plays a crucial role in molecular and cellular responses to DNA damage and in the maintenance of genome stability. It is downstream of ataxia telangiectasia mutated (ATM) damage signaling pathway and there is an emerging role of the transcription factor FoxO3a, a regulator of a variety of other pathways, in activating this signaling. We asked whether H2AX may feedback to FoxO3a to affect respective FoxO3a-dependent pathways. We used a genetically matched pair of mouse embryonic fibroblast H2AX(+/+) and H2AX(-/-) cell lines to carry out comprehensive time-course and dose-response experiments and to show that the expression of several FoxO3a-regulated genes was altered in H2AX(-)(/-) compared to H2AX(+/+) cells at both basal and irradiated conditions. Hspa1b and Gadd45a were down-regulated four- to five-fold and Ddit3, Cdkn1a and Sod2 were up-regulated 2-3-fold in H2AX(-/-) cells. Using the luciferase reporter assay, we directly demonstrated that transcriptional activity of FoxoO3a was reduced in H2AX(-/-) cells. FoxO3a localization within the nuclear phospho-ATM (Ser1981) foci in irradiated cells was affected by the H2AX status, as well as its posttranslational modification (phospho-Thr32). These differences were associated with genomic instability and radiosensitivity in H2AX(-/-) cells. Finally, knockdown of H2AX in H2AX(+/+) cells resulted in FoxO3a-dependent gene expression patterns and increased radiosensitivity that partially mimicked those found in H2AX(-/-) cells. Taken together, our data suggest a role for FoxO3a in the maintenance of genome integrity in response to DNA damage that is mediated by H2AX via yet unknown mechanisms.